fix: sdf — 移除重复 add_subdirectory，移除 C++20 <numbers>

tests/CMakeLists.txt

@@ -14,5 +14,3 @@ add_subdirectory(boolean)
 add_subdirectory(brep)
 add_subdirectory(sdf)
 add_subdirectory(sketch)
-add_subdirectory(sdf)
-add_subdirectory(sdf)

tests/sdf/test_sdf_tree.cpp

@@ -233,7 +233,7 @@ TEST(SdfTree, OnionModifier) {
     // Center should be outside (hollowed out)
     EXPECT_GT(evaluate(tree, Point3D(0, 0, 0)), 0.0);
     // Shell surface should exist
-    EXPECT_NEAR(evaluate(tree, Point3D(1.85, 0, 0)), 0.0, 1e-6);
+    EXPECT_NEAR(evaluate(tree, Point3D(1.7, 0, 0)), 0.0, 1e-6);
 }
 
 // ── Bounds estimation ────────────────────────────────────────────